Ceramic materials having high hot hardness for cutting tools are suitable for machining of metals having high hardness, high tensile strength and low thermal diffusivity at high temperature, and particularly suitable for machining self-hardening materials such as nickel- or cobalt-based heat resistant super alloys (HRSA).
Many silicon nitride-based materials for cutting tools are produced using aluminum oxide (Al2O3) as a sintering aid. Aluminum and oxygen can replace silicon and nitrogen, respectively, in the crystal structure of silicon nitride, thereby forming a SiAlON ceramic. The SiAlON ceramic consists of Si—Al—O—N and can be often additionally stabilized by a cation Men+ wherein Me is selected from a large number of rare-earth metals and lanthanides with suitable ionic radius (r<1.0 Å), such as Yb, Dy, Lu, Li, Ca, Mg, or Sc.
Many SiAlON phases have been detected and characterized, as disclosed in Izhevskiy V A, Genova L A, Bressiani J C and Aldinger F., “Progress in SiAlON ceramics”, J. Eur. Ceram. Soc. 20, 2275˜2295 (2000), but predominant phases for cutting tool materials remain an α-SiAlON phase, RxSi12−(m+n)Al(m+n)OnN(16−n), wherein m is greater than 1.0 and less than 2.7, n is less than 1.2 and R is selected from the aforementioned metals and lanthanides having an ionic radius less than 1.0 Å, and β-SiAlON, Si6−zAlzOzN(8−z) wherein z is greater than 0 and less than 4.2.
In addition to stabilizing the α-SiAlON phase, the metal ions may act as catalysts for creating SiAlON crystals upon sintering. The metal ion facilitates formation of elongated SiAlON particles, usually in the β phase, but elongated particles of α-SiAlON are also formed (see Fang-Fang X, Shu-Lin W, Nordberg L-O and Ekstreom T, “Nucleation and Growth of the Elongated α′-SiAlON”, J. Eur. Ceram. Soc. 17(13) 1631-1638 (1997))
SiAlON materials may include an α-SiAlON phase and a β-SiAlON phase, and may further contain silicon carbide particles dispersed throughout the SiAlON matrix (see U.S. Pat. No. 4,829,791).
U.S. Pat. No. 5,370,716 to Mehrotra et al. discloses a SiAlON material comprising a β-SiAlON phase having a high z value. The β-SiAlON phase has a structure of Si6−zAlzOzN(8−z) wherein z is greater than 1 and less than 3.
U.S. Pat. No. 6,124,225 to Tien et al. discloses a SiAlON ceramic material having a high proportion of α-SiAlON. Tien et al. lists Nd, Sm, Gd, Dy, Yb and Y and mixtures thereof, as additives, and Gd as a preferred additive. In a preferred embodiment, a starting silicon nitride powder contains about 95% by weight of α-silicon nitride. The '225 patent to Tien et al. does not appear to be directed to a SiAlON ceramic produced from a starting powder mixture comprising a starting silicon nitride powder which contains either no or a small amount (that is, an amount having a lower limit of 0% by weight and an upper limit of about 1.6% by weight) of β-silicon nitride.
α-SiAlON can be formed using additives, as disclosed in an article of Nordberg et al. entitled “Stability and Oxidation Properties of RE-α-Sialon Ceramics (RE=Y, Nd, Sm, Yb)” (J American Ceramic Society 81 [6] pp. 1461-70 (1998)). In an embodiment, only one kind of rare-earth element (for example, Nd, Sm, or Yb) is used. The article disclosed SNE-10 (UBE) as the starting silicon nitride powder. That is, the article does not appear to address a SiAlON ceramic produced from a starting powder mixture comprising a starting silicon nitride powder which contains either no or a small amount (that is, an amount having a lower limit of 0% by weight and an upper limit of about 1.6% by weight) of β-silicon nitride.
It is preferable to provide an improved SiAlON material for cutting tool application which exhibits improved metal cutting performance properties including hardness, Young's Modulus, fracture toughness, thermal conductivity and thermal shock resistance, although current ceramic cutting tools exhibit properties (for example, hardness and fracture toughness) suitable for use as cutting tools. The same is applied to SiAlON wear components in that it is preferable to provide an improved SiAlON material for wear resistance application which exhibits improved performance although current SiAlON wear components exhibit satisfactory properties (for example, hardness and fracture toughness).
In this regard, when the powder mixtures are sintered to produce the SiAlON material, crystalline phases can be formed in the grain boundaries between α-SiAlON grains and β-SiAlON grains. An increase in content of crystalline phases in the grain boundaries can cause a decrease in fracture toughness of the SiAlON material. Thus, it is preferable to provide a SiAlON material which has a minimal amount of grain boundaries and crystalline phases formed therein.
A SiAlON ceramic material having a high hardness is advantageous for use in certain applications such as a cutting insert and a wear part. Generally, a SiAlON ceramic material having a finer grain size shows a higher hardness. Accordingly, it is preferable to provide a SiAlON ceramic material which has a finer grain size and thus a higher hardness.